Real-time tracking of ionic nano-domains under shear flow

Clodomiro Cafolla,Kislon Voïtchovsky

doi:10.1038/s41598-021-98137-y

Abstract

The behaviour of ions at solid–liquid interfaces underpins countless phenomena, from the conduction of nervous impulses to charge transfer in solar cells. In most cases, ions do not operate as isolated entities, but in conjunction with neighbouring ions and the surrounding solution. In aqueous solutions, recent studies suggest the existence of group dynamics through water-mediated clusters but results allowing direct tracking of ionic domains with atomic precision are scarce. Here, we use high-speed atomic force microscopy to track the evolution of Rb+, K+, Na+ and Ca2+ nano-domains containing 20 to 120 ions adsorbed at the surface of mica in aqueous solution. The interface is exposed to a shear flow able to influence the lateral motion of single ions and clusters. The results show that, when in groups, metal ions tend to move with a relatively slow dynamics, as can be expected from a correlated group motion, with an average residence timescale of ~ 1–2 s for individual ions at a given atomic site. The average group velocity of the clusters depends on the ions’ charge density and can be explained by the ion’s hydration state. The lateral shear flow of the fluid is insufficient to desorb ions, but indirectly influences the diffusion dynamics by acting on ions in close vicinity to the surface. The results provide insights into the dynamics of ion clusters when adsorbed onto an immersed solid under shear flow.

Highlights

The behaviour of ions at solid–liquid interfaces underpins countless phenomena, from the conduction of nervous impulses to charge transfer in solar cells
Atomic force microscopy (AFM) can image single ions adsorbed at various solid–liquid interfaces[7,17,30,33,48,49,50]
We use high-speed AFM (HS-AFM) to track the dynamics of ionic nano-domains adsorbed at the mica-water interface when subject to a lateral shear flow of the liquid

Summary

Introduction

The behaviour of ions at solid–liquid interfaces underpins countless phenomena, from the conduction of nervous impulses to charge transfer in solar cells. Computer simulations can capture ions’ femto- to nanosecond dynamics[38,39], but with current capabilities, most investigations are limited both in number of atoms and in their full duration, rarely going beyond hundreds of nanoseconds[38,40] Key effects, such as water molecules dissociation, pH and mobility[39,41], are difficult to account for. The relatively slow temporal resolution of standard AFM (typically 30–100 s per image) limits its use for obtaining dynamical information, but multiple improvements to the technique over the last decade have enabled the emergence of high-speed AFM (HS-AFM). We use HS-AFM to track the dynamics of ionic nano-domains adsorbed at the mica-water interface when subject to a lateral shear flow of the liquid. While higher temporal resolution is technically possible, we aim to strike a compromise

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Conclusion